An ever-increasing number of relatively cheap, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technologies are described in detail in the 802.11 IEEE Standard, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g (2003), as well as the IEEE Standard 802.11n now in the process of being adopted, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or more effective throughput, making them a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” as well as the 802.11n standards achieve their high data transmission rates using Orthogonal Frequency Division Modulation or OFDM encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. In a general sense, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system.
Transmitters used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n standards as well as other standards such as the 802.16a IEEE Standard, typically perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n and 802.16a IEEE standards typically include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. The digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconverting, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain signal. Thereafter, the baseband processor performs demodulation and frequency domain equalization (FEQ) to recover the transmitted symbols, and these symbols are then processed with an appropriate FEC decoder, e.g. a Viterbi decoder, to estimate or determine the most likely identity of the transmitted symbol. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
In wireless communication systems, the RF modulated signals generated by the transmitter may reach a particular receiver via a number of different propagation paths, the characteristics of which typically change over time due to the phenomena of multi-path and fading. Moreover, the characteristics of a propagation channel differ or vary based on the frequency of propagation. To compensate for the time varying, frequency selective nature of the propagation effects, and generally to enhance effective encoding and modulation in a wireless communication system, each receiver of the wireless communication system may periodically develop or collect channel state information (CSI) for each of the frequency channels, such as the channels associated with each of the OFDM sub-bands discussed above. Generally speaking, CSI is information describing one or more characteristics of each of the OFDM channels (for example, the gain, the phase and the SNR of each channel). Upon determining the CSI for one or more channels, the receiver may send this CSI back to the transmitter, which may use the CSI for each channel to precondition the signals transmitted using that channel so as to compensate for the varying propagation effects of each of the channels.
To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known to use multiple transmission and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the 802.11n IEEE Standard now being adopted. As is known, the use of MIMO technology produces significant increases in spectral efficiency, throughput and link reliability, and these benefits generally increase as the number of transmission and receive antennas within the MIMO system increases.
In particular, in addition to the frequency channels created by the use of OFDM, a MIMO channel formed by the various transmission and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different SNRs at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
It is known that the use of multiple spatial channels in a MIMO system significantly increases throughput of the system as multiple streams of data can be sent through the system simultaneously. Thus, the use of multiple antennas within the MIMO system allows the use of multiple spatial streams, each of which includes streams of encoded data that are independently modulated and transmitted from the antennas. Generally speaking, the number of spatial streams is less than or is equal to the number of transmit antennas. When the number of transmit antennas is equal to the number of spatial streams, the modulated symbols of the spatial stream are spread evenly across the transmission antennas (i.e., one spatial stream per antenna) and are transmitted in parallel from the transmission antennas. However, when the number of spatial streams is less than number of transmission antennas, a spatial spreading matrix is used to map the spatial streams onto the transmission antennas to provide for maximum usage of the transmission antennas and thus maximum throughput. Generally speaking, it is possible to use a different spatial spreading matrix for each of the separate or possible tones or combinations of tones of the modulation system (wherein each tone relates to a different one of the possible symbols) to thereby allocate or to provide a spatial spreading matrix for use with the system that is optimally configured to send each of the separate tones. However, this system requires storing of a significant number of different spatial spreading matrices based on the tones, the number of tones and the combinations of tones sent in the system, and thus requires a significant amount of memory to store the spatial spreading matrixes. This requirement is especially true in the larger systems that have a significant number of spatial streams and/or transmission antennas. Generally speaking, in these systems as well as in other systems, the spatial spreading matrix is chosen to have orthogonal columns, so as to allocate the same amount of energy in each spatial stream.
However, from an implementation perspective, it is easier to design a transmission system having a single spatial spreading matrix that is used for all of the possible tones or combinations of tones. In the past, it was known and generally accepted to use a discrete Fourier transform (DFT) unity matrix as the spatial spreading matrix when the number of spatial streams was the same as the number of transmission antennas. Moreover, it has been typical to use only a portion of the DFT unity matrix (determined for the number of transmission antennas being used) when the number of spatial streams is less than the number of transmission antennas. Thus, in a system in which two spatial streams are transmitted simultaneously through three transmission antennas, two columns of the three-by-three DFT unitary matrix might be used as the spatial spreading matrix.
While such a system is generally acceptable when the data being sent within the separate signal streams is uncorrelated, and thus is random with respect to one another, problems can arise when the data being sent between the separate signal streams is correlated, which frequently occurs in communication systems that have predefined headers such as in communication systems using the 802.11(x) standards. In this case, significant portions of the symbol bit streams within the two separate spatial streams are correlated with one another, and can result in one of the transmission antennas transmitting a significantly higher power than the other transmission antennas. Thus, for example, when three transmission antennas are used to transmit two separate bit or symbol streams, and the two separate bit or symbol streams have the identical data, the first transmission antenna might end up being used to transmit at, for example, four times the power as the other two transmission antennas.
This unequal power situation causes a problem because the power amplifiers used in transmission systems generally have non-linear characteristics when operated well outside of a normal operating range. Thus, if two transmission antennas transmit at a particular power which is within the normal operating range of the associated power amplifiers, and one of the transmission antennas transmits at four times that power, this last transmission antenna may operate in a non-linear or abnormal region of the power amplifier, causing the power amplifier of this last transmission antenna to fail to properly amplify the signal as compared to the amplification provided by the other two transmission paths. These non-linearities result, on the receiver side of the transmission system, in distortions within the data, which leads to possible improper decoding of symbols at the receiver side, resulting in high and possibly unacceptable data error rates.